1. Field of the Invention
The present invention generally relates to communications systems and, more particularly, to a technique for orthogonalizing transmission systems which reuse frequency on dual polarizations.
2. Description of the Prior Art
Using two orthogonal polarizations simultaneously effectively doubles the bandwidth of a communications system. Systems using this technique are referred to as frequency reuse systems and are of particular interest in communications satellite systems. However, due to the polarization characteristics of the spacecraft antenna and the earth station antenna such as non-ideal feed axial ratio, antenna depolarization, and the like, there exists a certain amount of "static" cross-coupling of signals. In addition, such factors as Faraday rotation, spacecraft orientation and depolarization of signals due to rainfall and other atmospheric effects, there is a rather large coupling of signals which is "dynamic" (i.e., time-varying) in nature. Both static and dynamic cross-polarization interference act to seriously degrade the performance of frequency reuse systems.
It has been suggested by T.S. Chu, "Restoring the Orthogonality of Two Polarizatons in Radio Communications Systems, I," The Bell System Technical Journal, Vol. 50, No. 9, November 1971, at pages 3063 to 3069, that by using a differential phase shifter and a differential attenuator, the orthogonality can be recovered by the transformation of two nonorthogonal elliptically polarized waves into two orthogonal polarizations. The system proposed by T. S. Chu is implemented with a lossy compensating network at only the transmitting or receiving end of the transmission link and not at both. In fact, it can be shown that, in general, orthogonalization networks inserted in the system at either the transmitting or receiving end, but not both, will be lossy.